Inrush current limiting circuit

ABSTRACT

In various embodiments a circuit is provided including: an input terminal to receive an input voltage; a switch, a first controlled input of which being coupled to the input terminal; an inductor, a first terminal of which may be coupled in series to a second controlled input of the switch; a freewheeling diode, wherein a first diode terminal may be coupled with the second controlled input of the switch and with the first terminal of the inductor, and wherein a second diode terminal may be coupled with a reference potential; a capacitor coupled with a second terminal of the inductor; and a controller configured to operate the switch and the inductor in continuous current mode to charge the capacitor.

TECHNICAL FIELD

Various embodiments relate to an inrush current limiting circuit.

BACKGROUND

Many electronic devices draw high instantaneous input currents whenfirst switched on. Those currents are referred to as inrush currents andthey not only complicate the design of, for example, overcurrentprotection devices within the electronic device, but they may also causeactual damage to the electronic device as those inrush currents mayexceed the normal steady state operating current of the electronicdevice by orders of magnitude.

One of the fields of electronics where high inrush currents may have tobe dealt with are airbag systems. Airbag systems used nowadays mostlyinclude an energy reserve capacitor which may be used to store chargesfor the case when for some reason, for example due to a crash, theelectrical connection to the vehicle battery is lost. Therefore, thecapacitance of the reserve capacitor is usually quite large, for exampleit may be a few tens of millifarad, for example 10 mF, in the exemplarycase of a charge amounting to 330 millicoulomb which is stored at anoperating voltage of 33V. With a reserve capacitor dimensioned properly,the airbag module may thus be provided with energy therefrom and be ableto continue its proper operation. Thus the airbag may be deployed evenwhen the connection to the battery is lost in case of an accident.However, the presence of the reserve capacitor with its big capacitancemay be problematic with respect to the inrush current which may occurwhen the ignition of the car is enabled, triggering the start up of theairbag module. When the ignition key is inserted and turned as the caris to be started, a high inrush current may be drawn by the empty (i.e.discharged) reserve capacitor. Costumers require an inrush current phasewhich is fast and controlled. However, serious problems have beenreported in the past which are linked to inappropriate handling of theinrush current, even resulting in inadvertent deployment of the airbagwhen allowing the reserve capacitor to be charged in an uncontrolledmanner.

With remotely controlled door locks entering the automotive market manycar manufacturers choose to turn on the airbag module prior to theignition key being inserted into the ignition, namely when the remotecontrol button unlocking the car doors is pressed. This feature mayrequire a so called “wake switch” which permits the airbag module to beinitiated prior to turning on the car in the traditional way via itsignition switch. However, the “wake switch” in off-state may cause avery low quiescent current consumption when in sleep mode.

The two previously described requirements are typically providedseparately from one another in airbag modules. The capacitor inrushcurrent limiter may be provided in the form of an NFET (n-channel fieldeffect transistor) with a current shunt resistor. The current flowingthrough the NFET is monitored via the shunt resistor and an overcurrentregulator may be used to control the gate of the NFET such that the NFETis driven into a more or less conducting state in correspondence to thecurrent flowing therethrough. However, this solution requires anadditional shunt resistor and an NFET gate driver including anovercurrent regulator. Both components are space consuming and costly.Instead of the separate additional shunt resistor, a special sense-NFETmay be used which is provided with an internal current sensingfunctionality. Those approaches may have the further disadvantage thatthe NFET acting as a linear controller may get and therefore has todissipate more power in the event of a high inrush current. Furthermore,as the current is limited, the main microcontroller of the airbag modulemay start its operation with a delay on the order of 100 milliseconds ormore.

A further setup configured to provide capacitor inrush currentlimitation may include a current limiting resistor coupled between areference potential and one side of the reserve capacitor, wherein aswitch, e.g. a NFET, may be coupled in parallel to the current limitingresistor. The current limiting resistor is bypassed with the NFET duringnormal operation of the airbag module, i.e. after the inrush currentphase when the car is already running. In a yet further setup, a boostconverter is arranged between the battery and the reserve capacitor. Acurrent limiting resistor is arranged between the boost capacitor of theboost converter and the reserve capacitor, wherein a diode is coupled inparallel to the current limiting resistor in order to provide a reversepath in case the battery is lost and the whole infrastructure has to beprovided with energy from the reserve capacitor. In this scenario, theboost capacitor and the reserve capacitor have to be provided asseparate entities, adding to the space and cost requirement of thearrangement.

The wake switch functionality may be implemented by providing a wakeswitch, for example a PFET (p-channel FET) between the battery and theairbag module, wherein the wake switch may be turned on and off by meansof a respective signal from the car's electronics, for example by meansof a respective CAN (controller area network) signal. The PFET may bereplaced by an NFET, which is roughly only half the size of a PFET, butrequires a gate voltage lying above the source voltage such that acharge pump may have to be used. However, both setups are not providedwith a soft start function.

In a further approach, the wake-switch functionality and the capacitorinrush current limitation functionality may be provided in one commonintegrated concept. This concept is based on an intelligent boostconverter rectifier diode, which consists of a NFET based face to faceconfiguration to eliminate the existing body diodes and to achieve an“ideal switch” like a relay. The two rectifier diodes provide isolationof the reserve capacitors from the battery in sleep mode (off-state) andfurther an inrush current limitation for power-on of the main operatingmodule and rectifying functionality for the boost converter operation.

SUMMARY

In various embodiments a circuit is provided including an input terminalto receive an input voltage; a switch, a first controlled input of whichbeing coupled to the input terminal; an inductor, a first terminal ofwhich may be coupled in series to a second controlled input of theswitch; a freewheeling diode, wherein a first diode terminal may becoupled with the second controlled input of the switch and with thefirst terminal of the inductor, and wherein a second diode terminal maybe coupled with a reference potential; a capacitor coupled with a secondterminal of the inductor; and a controller configured to operate theswitch and the inductor in continuous current mode to charge thecapacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. The drawings are not necessarilyto scale, emphasis instead generally being placed upon illustrating theprinciples of the invention. In the following description, variousembodiments of the invention are described with reference to thefollowing drawings, in which:

FIG. 1 shows an embodiment of the circuit according to variousembodiments;

FIG. 2 shows a further embodiment of the circuit according to variousembodiments;

FIG. 3 shows a diagram explaining the operation of the circuit accordingto various embodiments;

FIG. 4 shows a flow diagram explaining a method to operate the circuitaccording to various embodiments; and

FIG. 5 shows the circuit according to various embodiments.

DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practiced.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration”. Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

The word “over” used with regards to a deposited material formed “over”a side or surface may be used herein to mean that the deposited materialmay be formed “directly on”, e.g. in direct contact with, the impliedside or surface. The word “over” used with regards to a depositedmaterial formed “over” a side or surface, may be used herein to meanthat the deposited material may be formed “indirectly on” the impliedside or surface with one or more additional layers being arrangedbetween the implied side or surface and the deposited material.

In FIG. 1 a circuit 100 according to various embodiments is shown. Thecircuit 100 according to various embodiments includes an input 102. Theinput 102 may be connected to a battery (not shown), for example to abattery of a vehicle such as a car. However, the input 102 may becoupled to any other suitable energy source which may act as a powersupply and power the circuit 100. The input 102 of the circuit 100according to various embodiments may be connected to a first controlledterminal of a switch 106 via a first diode 104. The first controlledterminal may be configured as a source/drain terminal and the switch 106may be configured as a FET (field effect transistor), for example a PFET(p-channel FET). A second controlled terminal of the switch 106 may becoupled to a first side of an inductor 110. A second side of theinductor 110 may be coupled to an output 114 of the circuit 100according to various embodiments via a second diode 112. One terminal ofa third diode 108 may be coupled to the electrical path between theswitch 106 and the inductor 110, the other terminal of the third diode108 may be coupled to a reference potential, for example the groundpotential. One side of a first capacitor 116 may be coupled to theelectrical path between the second diode 112 and the output 114 of thecircuit 100. The other side of the first capacitor 116 may be coupled tothe reference potential. One side of a second capacitor 118 may becoupled to the electrical path between the second diode 112 and theoutput 114 of the circuit 100. The other side of the second capacitor118 may be coupled to the reference potential. Therefore, the firstcapacitor 116 and the second capacitor 118 may be coupled in parallel.However, the first capacitor 116 and the second capacitor 118 may beprovided as one capacitor with a capacitance which for examplecorresponds to the sum of the capacitance of the first capacitor 116 andthe capacitance of the second capacitor 118. The circuit 100 accordingto various embodiments may further include a controller 120. Thecontroller 120 may for example be configured as an ASIC (applicationspecific integrated circuit) and it may include or be any kind ofintegrated circuit, for example a microchip, a RISC (reduced instructionset computer) microprocessor and/or a CISC (complex instruction setcomputer) microprocessor.

The controller 120 may have a first terminal VIGN_IN which may becoupled to the second controlled terminal of the switch 106. The firstterminal VIGN_IN may be used to sense the voltage at the output of thefirst switch 106, i.e. at its second controlled terminal. The controller120 may have a second terminal WAKESW_CTRL which may be coupled to thecontrol terminal of the switch 106, for example to the gate terminal ofthe switch 106 configured as a PFET. The second terminal WAKESW_CTRL maybe used to control the conductivity of the switch 106, i.e. to set theswitch 106 into a more or less conductive state and to generally switchon and switch off the switch 106. The controller 120 may have a thirdterminal VBATT_MON which may be coupled to the electrical path betweenthe first diode 104 and the switch 106. The third terminal VBATT_MON maybe used by the controller 120 to monitor the input voltage, i.e. thevoltage provided at the input 102 of the circuit 100 according tovarious embodiments which may be derived from a battery, for example.The controller 120 may have a fourth terminal WAKE_EN at which a signalmay be provided to the controller 120, for example a signal form amicrocontroller or a CAN (controller area network) signal. The signalprovided at the fourth terminal WAKE_EN to the controller 120 may beused to wake up the controller 120, i.e. activate the controller 120when it is inactive or in sleep mode. The controller 120 may have afifth terminal which may include a pad which may be connected to thereference potential. The controller 120 may have a sixth terminalVBCK2_IN and a seventh terminal VBCK1_IN, both of which may be coupledto the electrical path between the second diode 112 and the output 114of the circuit 100. The controller 120 may have an eighth terminalVBST_FBK which may be coupled to the electrical path between the seconddiode 112 and the output 114 of the circuit 100. The controller 120 mayhave a ninth terminal BST_D which may be coupled to the electrical pathbetween the inductor 110 and the second diode 112.

The circuit 100 according to various embodiments may be configured toprovide inrush current limiting functionality. The circuit 100 may offerthe advantage of using components which may be already present in anairbag module in a car, such as the switch 106 which may fulfil thefunction of a wake switch and the inductor 110 which may fulfil thefunction of the boost inductor. Those two components just listed, thethird diode 108 and the first capacitor 116 which may correspond to aboost capacitor, form a buck converter. The switch 106 may be used topower the system (the circuit 100 and components or circuits connectedto its output), i.e. to establish low resistance path between the input102 of the circuit 100 according to various embodiments and its output114. The switch 106 may be for example closed (i.e. rendered conductive)to provide a low resistance path between the battery and the firstcapacitor 116 and the second capacitor 118. In the circuit 100 accordingto various embodiments the switch 106, which may be integrated with theother components of the circuit 100 or may be an external switch, may bealso used for current limitation.

During the inrush current phase, i.e. during the phase in which thecircuit 100 is enabled after being inactive and hence the firstcapacitor 116 and/or the second capacitor 118 are free of charges, thecurrent flowing through the switch 106 is monitored. When the currentduring the inrush current phase reaches a predefined upper threshold,the switch 106 may be turned off (i.e. rendered non-conductive) by thecontroller 120 for a predefined period of time. This off-time may be afixed period of time as it is the case in a PFM (pulse-frequencymodulation) driving scheme or it may just as well be a variable periodof time as it would be the case in a PWM (pulse-width modulation)driving scheme. During that time, the current will continue to flowthrough the third diode 108 which then fulfils the role of afreewheeling-diode into the first capacitor 116 and the second capacitor118. Therefore, the cathode of the third diode 108 may be coupledbetween the second controlled terminal of the switch 106 and one side ofthe inductor 110, the anode of the third diode 108 may be coupled to thereference potential. After the current has decreased to a predefinedlower threshold, the switch 106 is closed again such that the currentmay flow from the input 102 of the circuit 100 to its output 114. Theupper threshold value of the inrush current may be defined by themaximum rated current the circuit 100 may withstand without gettingdegraded or damaged. By successive switching on and switching off of theswitch 106, an average constant current may be provided to the firstcapacitor 116 and the second capacitor 118. The capacitors 116, 118 maybe charged with a controlled constant (on average) inrush current. Inother words, during the inrush current phase, the circuit 100 accordingto various embodiments behaves as a buck converter in currentlimitation, wherein the inrush current has a triangular shape which istypical of buck converters due to the inductive load switching. In thecircuit 100 according to various embodiments as presented in FIG. 1 thefirst capacitor 116 may be a boost capacitance. The switch 106functioning as a wake switch may be used as a current limiting componentduring the inrush current phase. The switch 106 may be operated in aswitched manner during the inrush current phase to limit the maximuminrush current until the voltage across the first capacitance 116 and/orthe second capacitance 118 is equal to the voltage at the input 102 ofthe circuit 100 according to various embodiments. The voltage applied tothe input 102 of the circuit 100 may, for example, correspond to thevoltage of a battery connected to the input 102 of the circuit 100.After the capacitors 116, 118 are charged (and hence the inrush currentphase is over and the current flow through the switch 106 has reduced tothe circuit's steady state current flow), the switch 106 may remainpermanently switched on (i.e. in a state of low or almost noresistance). The voltage at the output 114 of the circuit 100 accordingto various embodiments may then correspond to approximately the voltageprovided at the input 102 thereof, for example the voltage provided by abattery coupled to the input 102 reduced by the forward voltage drop ofthe first diode 104 and the second diode 112.

Since the switch 106 is operated in a switched manner during the inrushcurrent phase, the power dissipation may be very low (as no power isdissipated in the switch 106 when no current is flowing therethrough).In other words, the energy dissipated by the switch 106 corresponding tothe heat produced by the switch 106 may be very moderate. Therefore, theswitch 106 may be integrated (for example provided on the same substrateor wafer as the rest of the components included in the circuit 100).Alternatively, the switch 106 may be provided separated from thecontroller 120, for example if the circuit 100 according to variousembodiments is to handle very high inrush currents. The switch 106acting as a wake switch may be used to ensure the flow of a controlledinrush current, such that a fast but well controlled inrush phase may bepermitted during which the energy reserve capacitor may be charged fast.As noted above, the first capacitor 116 and the second capacitor 118 maybe combined and thus provided as a single component which may result inreduced costs of the circuit 100 according to various embodiments andits more compact size.

The efficiency of the inrush current phase is determined by switchinglosses just as it would be the case in an ordinary buck converter. Byoperating the switch 106 in a switched manner during the inrush currentphase, over-heating of the switch 106 which is known to take place inlinear operating regulating pass-devices may be prevented. Thus, theswitch 106 may be safely integrated with the rest of the circuit 100according to various embodiments. Providing the switch 106 in anintegrated fashion may simplify sensing of the current through theswitch and also the sensing of its temperature to ensure that it doesnot overheat during the controlled inrush current phase. However, sincethe switch 106 may have to carry a significant current during the inrushcurrent phase, it may be configured as a switch with a low on-stateresistance R_(DSON) and with a capability to conduct high currents.Therefore, the switch 106 may be provided in the form of an externaldiscrete device.

As stated before, the switch 106 may be also be provided in the form ofan external switch. In that case, however, the current sensing circuitmay become more complicated. For example, the voltage across theinductor 110 may be sensed in order to further process the current.

In automotive applications (including functional safety tasks) where thewake switch, i.e. the switch 106, is provided, the supply line to theignition key switch (contact 15 according to DIN 72552 standard forautomobile electric terminal numbers) may be left out. The system may beinitiated or started by the wake switch alone (which is connected to thepermanent battery supply (contact 30 according to DIN 72552 standard forautomobile electric terminal numbers)) when a corresponding signal isreceived by the controller 120, for example a CAN message specifyingthat the controller 120 is to power up and to activate the switch 106.This may further simplify the design of the inrush current limitingcircuit which may be for example used in an airbag module.

In FIG. 2 a further implementation of the circuit according to variousembodiments is shown. The circuit 200 shown in FIG. 2 may be used toprovide an accurate current limitation in the case of the switch 106being provided externally, i.e. not being integrated with the rest ofthe components of the circuit 200 according to various embodiments onthe same substrate. In that case, an external sense resistor may be usedor the wake switch (switch 106) needs to be integrated such that thesensing of the current flowing therethrough may be performed or acomplex current sensing scheme based, for example, on integrating theinductor current may be employed. If no accurate current limitation isprovided, the inductor 110 may need to be designed larger so that itdoes not saturate or overheat during the inrush current phase.

The circuit 200 according to various embodiments shown in FIG. 2 isbased on the circuit 100 shown in FIG. 1. The same components shared byboth circuits carry the same reference numbers and will not be describedagain in the context of the circuit 200 shown in FIG. 2. Only newcomponents of the circuit 200 of FIG. 2 which are not present in thecircuit 100 shown in FIG. 1 will be described.

The circuit 200 according to various embodiments may include a secondswitch 202 which may be configured as an NFET, for example, and mayfulfil the function of a low-side switch of a boost converter. The firstcontrolled terminal, for example a first source/drain terminal, of thesecond switch 202 may be coupled to the electrical path between theinductor 110 and the second diode 112, a second controlled terminal, forexample the second source/drain terminal, of the second switch 202 maybe coupled to the reference potential via a sense resistor 204. Thecontroller 120 may further include a tenth terminal BST_SNSN which maybe coupled to the side of the sense resistor 204 which is coupled to thereference potential, for example the ground potential. The controller120 may further include an eleventh terminal BST_SNSP which may becoupled to the side of the sense resistor 204 which is coupled to thesecond switch 202. The controller 120 may further include a twelfthterminal BST_G which may be coupled to the control terminal, for examplethe gate terminal, of the second switch 202. The controller 120 mayinclude an internal switch 206, which may be configured as an NFET. Afirst controlled terminal of the internal switch 206, for example afirst source/drain terminal, may be coupled to the first controlledterminal of the second switch 202. A second controlled terminal of theinternal switch 206, for example the second source/drain terminal, maybe coupled to the reference potential, for example the ground potential.The control terminal of the internal switch 206, for example the gateterminal thereof, may be coupled to a boost control and monitoringcircuit 208 included in the controller 120. Also, the tenth terminalBST_SNSN, the eleventh terminal BST_SNSP and the twelfth terminal BST_Gwhich are provided in the controller 120 may be coupled to the boostcontrol and monitoring circuit 208. The second switch 202 and the senseresistor 204 may be external components with respect to the controller120 whereas the internal switch 206 may be provided within thecontroller 120. The switch 106, as in the previous exemplary embodimentof the circuit 100 shown in FIG. 1, may be integrated or it may beprovided externally.

As in the embodiment of the circuit 100 according to various embodimentsshown in FIG. 1, the embodiment shown in FIG. 2 makes use of alreadyexisting necessary components such as the switch 106, the inductor 110and the second switch 202 as well as the current limitation scheme ofthe second switch 202 and its controller 120 to provide a fast andcontrolled charging of the capacitor(s) provided at the output 114 ofthe circuit 200.

During the inrush current phase, the second switch 202 may be switchedon and switched off by the controller 120 synchronously with the switch106 such that from a functional point of view a non-inverting buck-boostconverter is formed. The second switch 202 may be seen to represent theboost switch. The internal switch 206 is equivalent to the second switch202 in its function, however it is provided inside the controller 120.It may be seen as a redundant switch which may be used in case thesecond switch 202 is damaged or it may be used when the second switch202 and the sense resistor 204 are not provided.

In the circuit 200 according to various embodiments it is possible toreuse the current limitation functionality of the boost converterincluded in the circuit 200 together with the controller 120 operatingthe boost converter. As the AC (alternate current) model of the circuitaccording to various embodiments is similar, it is possible to use thecontroller 120 during an operation mode, in which the converter includedin the circuit 200 according to various embodiments is no longer a boostconverter but a non-inverting buck-boost converter. As mentioned above,the non-inverting buck boost-converter may be realised by switching theswitch 106 on and off synchronously with the second switch 202. When theswitch 106 and the second switch 202 are switched on, a voltage equal tothe difference between the potential at the input 102 of the circuit 200and the reference potential is applied across the inductor 100 such thata current may flow therethrough and energy is stored in the inductor110. When the switch 106 and the second switch 202 are switched off, thecurrent flow from the input 102 of the circuit 200 through the inductor110 is interrupted and instead the current will flow through thefreewheeling diode 108 into the first capacitor 116 and the secondcapacitor 118, wherein the latter may for example represent the energyreserve capacitor.

The switch 106 and the second switch 202 in the circuit 200 according tovarious embodiments may be used in combination to implement a currentlimiting scheme. During the inrush current phase, the non-invertingbuck-boost converter may start its operation using the controller 120which may be referred to as the boost controller and which may controlboth switches, i.e. the switch 106 fulfilling the role of the wakeswitch and the second switch 202 fulfilling the role of the boost switch(or the internal switch 206 in case the second switch 202 is damaged ornot provided at all). The current flowing through the second switch 202may be sensed and monitored by the controller 120 by means of the senseresistor 204. The operation of the circuit 200 according to variousembodiments during inrush current phase is equivalent to the operationof the circuit 100 according to various embodiments shown in FIG. 1,except that instead of only the switch 106 being turned on and off forpredetermined periods of time the second switch 202 is turned on and offsynchronously with the switch 106 to provide a fast and controlledinrush current phase. As in the previous exemplary embodiment of thecircuit 100 shown in FIG. 1 the power dissipation in the switches ismoderate and hence they do not heat up significantly. Therefore, boththe switch 106 and the second switch 202 may be provided as integrateddevice within the circuit 200.

During the inrush current phase the energy reserve capacitor, forexample the second capacitor 118, is charged with a constant current(see FIG. 3, time period B in diagram 300). Once the voltage across thesecond capacitor 118 is equal to the voltage applied to the input 102 ofthe circuit 200, the inrush current phase is finished. This event, forexample, may be detected by a comparator which may be provided withinthe controller 120. After the inrush current phase is finished, theswitch 106 may remain switched on permanently and allow the converter tobehave as a boost converter, wherein only the second switch 202 thenfulfilling the role of the boost switch may be operated in a switchedmanner (see FIG. 3, time period C in diagram 300). Due to the switchingof the second switch 202, the heat generated in the switch 106 due toenergy dissipation may be reduced. Also, the efficiency of the system isincreased to the efficiency of a simple boost converter.

In diagram 300 in FIG. 3 the evolution of the current I_(L) through theinductor 110 is shown. The x-axis 302 denotes time in arbitrary units,the y-axis 304 denotes current in arbitrary units.

The graph 308 representing the current I_(L) through the inductor 110 issubdivided into three different phases. In a first phase A the currentI_(L) may be seen to decrease from zero to the upper threshold value.The beginning of the first phase A may represent the activation of anairbag module. During the first phase A, the switch 106 (and a furtherswitch, be it the second switch 202 (external) or the internal switch206, if present) remains switched on until the current I_(L) reaches anupper threshold peak value 310. The current I_(L) reaching the upperthreshold peak value 310 for the first time initiates a second phase Bduring which the switch 106 is operated in a switched manner, i.e.switched on and off for predefined periods of time. During the secondphase B, the circuit 100 according to various embodiments of FIG. 1operates as a buck converter, whereas the circuit 100 according tovarious embodiments of FIG. 2 operates as a non-inverting buck-boostconverter. However, the effect in both cases is equivalent. In bothcases a controlled inrush current phase is provided, wherein the energyreserve capacitor (e.g. the second capacitor 118) may be charged with acontrolled average inrush current resulting from the switched operationof the converter. In both cases, the converters may operate incontinuous conduction mode, i.e. the controlled inrush current may flowcontinuously through the inductor throughout the switching period. Theaverage current is indicated by the dashed line 306 in FIG. 3. Duringthe charging process of the energy reserve capacitor, the voltage acrossthat capacitor may be monitored by the controller 120. As soon as thevoltage across the energy reserve capacitor is equal to the voltageprovided at the input 102 of the circuit according to variousembodiments, the inrush current phase is over. This event marks the endof the second phase B and a third phase C may be initiated whichcorresponds to the steady state operation of the circuit according tovarious embodiments. During that phase, the energy reserve capacitor 118remains in its fully charged state and the switch 106 remains switchedon permanently, whereas in the circuit 200 according to variousembodiments shown in FIG. 2 the switch 106 remains switched onpermanently and the second switch 202 fulfilling the role of a boostswitch may be operated in a switched manner with a predefined dutycycle. In other words the circuit 200 according to various embodimentsshown in FIG. 2 behaves as a boost converter after the controlled inrushcurrent phase is finished.

In FIG. 4 a flow diagram 400 explaining a method to operate the circuitaccording to various embodiments is shown.

In a first step 402, during the inrush current phase when the circuitaccording to various embodiments is started and at least the energyreserve capacitor is not charged, the switch 106 may be operated in aswitched manner as to permit a fast and controlled inrush current phase.During that phase which may correspond to the first interval A and thesecond interval B in diagram 300 in FIG. 3, at least the energy reservecapacitor (e.g. the second capacitor 118) may be charged with a constantcurrent provided by the converter included in the circuit according tovarious embodiments. That constant current (represented by the dashedline 306 in FIG. 3) corresponds to the average current output by thecircuit which during this phase operates as a buck converter (in thecase of the circuit 100 according to various embodiments shown inFIG. 1) or as a non-inverting buck-boost converter (in the case of thecircuit 100 according to various embodiments shown in FIG. 1) incontinuous current mode (CCM).

In a second step 404, once the inrush current phase is finished, theswitch 106 remains in a switched on (i.e. conducting) state(corresponding to the third interval C in FIG. 3). The end of the inrushcurrent phase may be seen to occur with the event of the voltage acrossat least the energy reserve capacitor being equal to the voltageprovided at the input 102 of the circuit according to variousembodiments. The monitoring of the two voltages may be performed by thecontroller 120. Once the controller detects the voltage across at leastthe energy reserve capacitor being equal to the voltage provided at theinput 102 of the circuit according to various embodiments, it maydiscontinue driving the switch 106 in a switched manner. This may be thecase for both exemplary embodiments of the circuit shown in FIG. 1 andFIG. 2. In the case of the circuit 200 according to various embodimentsshown in FIG. 2, the circuit 200 may behave as a boost converter duringthe third interval C in diagram 300 of FIG. 3 which corresponds to aphase following the inrush current phase (represented by the firstinterval A and the second interval B in diagram 300).

The circuit according to various embodiments may be used as an inrushcurrent limiter in automotive applications, for example in airbagmodules. The energy reserve capacitor, which may correspond to thesecond capacitor 118, may have a capacitance in the range of a few tensof millifarad, for example 20 millifarad. The energy reserve capacitormay be provided in order to ensure proper operation of the airbag modulein case the battery is lost and cannot supply energy to the airbagmodule.

The electrical connection between the battery and the airbag module maybe for example damaged during an accident. The energy reserve capacitormay have to supply currents in the range of 30 amperes when all squibs,i.e. the components within the air bag inflator that begin thedeployment process of the airbag, are fired. In the circuit according tovarious embodiments the energy reserve capacitor (for example the secondcapacitor 118) and the boost capacitor (for example the first capacitor116) may be combined such that only one capacitance device needs to beprovided.

In general, the start up (i.e. the activation) of an airbag system in avehicle may last on the order of a few seconds, for example it may lastbetween 4 and 6 seconds. The circuit according to various embodimentsmay be used in the airbag module to provide maximum controlled inrushcurrent to permit a fast and simultaneously well controlled inrushcurrent phase such that the energy reserve capacitor may be charged asfast as possible, but in a controlled manner. The maximum controlledinrush current during that phase may correspond to the maximallyachievable current from the operation of the circuit according tovarious embodiments as a buck converter or as a non-inverting buck-boostconverter, depending on the actual implementation of the circuit. Sincethe wake switch (i.e. the switch 106) and the boost switch (i.e. theswitch 202 as an external boost switch or its internal version in theform of the internal switch 206) may be operated in a controlledswitched manner (i.e. they may be turned on and off for predeterminedtimes) instead of being linear regulators, the heat dissipation may bereduced. Therefore, both switching devices may be integrated.

The frequency at which at least one of the switches is switched duringthe controlled inrush current phase may be chosen to correspond to thefrequency of a pi-filter which is commonly used in automotiveapplications and may be provided to ensure that noise from the airbagmodule cannot spread onto the power rails connected with the battery ofthe vehicle and other electronic components thereof. The maximum dampingfrequency of pi-filters used in automotive applications is in the rangeof 300 kHz. Therefore, the switch 106 and the second switch 202 (ifpresent in the embodiment) may be switched at the same rate by using anappropriately configured PWM scheme such that the noise produced by theswitching scheme may be effectively filtered out by the pi-filter.However, the switching devices may be chosen to be switched at lower orhigher frequencies as well, for example at 600 kHz, which may enable thecircuit according to various embodiments to provide a higher averagecurrent. The pi-filter may be for example coupled between the energysource such as a battery and the input 102 of the circuit according tovarious embodiments. The pi-filter has been only mentioned by way ofexample due to its prevalent use in automotive applications and may beof course replaced by any other filter topology providing the desiredfiltering bandwidth.

The switch 106 may be further used as a safety switch. In the case of ahard shortcut which may be for example present inside the internalswitch 206 or inside the second switch 202, the switch 106 may berendered non-conducting and hence prevent the shorting of the powersource, for example a vehicle battery, to the reference potential. Inother words, the circuit according to various embodiments may haveinherent functional safety by virtue of the switch 106.

The circuit according to various embodiments may be further configuredto maintain the voltage applied to the at least one capacitor (i.e. theenergy reserve capacitor and/or the boost capacitor) in case the voltageapplied to the input 102 of the circuit according to various embodimentsexceeds its nominal value. This may be performed by operating thecircuit according to various embodiments in the same manner as duringthe controlled inrush current phase, only that in this case the controlscheme may be voltage based and not current based. In other words, thecontroller 120 may be configured to monitor the voltage applied to atleast one of the capacitors and switch on the switch 106 and the secondswitch 202 (if present) such that the voltage applied to the at leastone capacitor maintains its steady state value. In this way, thecapacitors may be protected from being “stressed” by a too high voltage.In automotive applications, the battery is usually configured to providea voltage of maximum 40V.

In FIG. 5, an implementation of the circuit according to variousembodiments is shown. The circuit 500 according to various embodimentsmay include an input terminal 502 to receive an input voltage, a switch504, a first controlled input of which may be coupled to the inputterminal 502, an inductor 510, a first terminal of which may be coupledin series to a second controlled input of the switch 504, a freewheelingdiode 508, wherein a first diode terminal may be coupled with the secondcontrolled input of the switch 504 and with the first terminal of theinductor 510, and wherein a second diode terminal may be coupled with areference potential 516, a capacitor 512 coupled with a second terminalof the inductor 510, and a controller 506 configured to operate theswitch 504 and the inductor 510 in continuous current mode to charge thecapacitor 512. The circuit 500 according to various embodiments mayfurther include an output terminal 514 to provide an output voltage.

In accordance with various embodiments, a circuit is provided which mayinclude an input terminal to receive an input voltage, a switch, a firstcontrolled input of which being coupled to the input terminal, aninductor, a first terminal of which is coupled in series to a secondcontrolled input of the switch, a freewheeling diode, wherein a firstdiode terminal may be coupled with the second controlled input of theswitch and with the first terminal of the inductor, and wherein a seconddiode terminal may be coupled with a reference potential, a capacitorcoupled with a second terminal of the inductor, and a controllerconfigured to operate the switch and the inductor in continuous currentmode to charge the capacitor.

In accordance with various further embodiments, the circuit may includea battery, wherein the input terminal may coupled to an output of thebattery.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch on the switch when the operationof the circuit is to be initiated.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch off the switch when the currentthrough the switch has reached a predetermined upper threshold value.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch on the switch when the currentthrough the switch has fallen to a predetermined lower threshold valuefrom a predetermined upper threshold value.

In accordance with various further embodiments of the circuit, thepredetermined lower threshold value may be smaller than thepredetermined upper threshold value.

In accordance with various further embodiments, the circuit may furtherinclude a voltage sensing circuit configured to determine the voltageacross the capacitor.

In accordance with various further embodiments of the circuit, thevoltage sensing circuit may be coupled to the controller.

In accordance with various further embodiments of the circuit, thevoltage sensing circuit may be an integral part of the controller.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch on the switch permanently whenthe voltage across the capacitor is substantially equal to the inputvoltage.

In accordance with various further embodiments of the circuit, as longas the voltage across the capacitor is smaller than the input voltage,the circuit may be configured to operate as a buck converter.

In accordance with various further embodiments, the circuit may furtherinclude at least one second capacitor which may be coupled in parallelto the capacitor. The at least one second capacitor may be configured asa tank capacitor.

In accordance with various further embodiments of the circuit, thecapacitance of the capacitor may be larger than 1 Millifarad.

In accordance with various further embodiments of the circuit, thecapacitance of the capacitor may be larger than the capacitance of thesecond capacitor.

In accordance with various further embodiments, the circuit may furtherinclude an input filter coupled to the input terminal

In accordance with various further embodiments of the circuit, thefilter may be configured as a pi filter.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch on and switch off the switch at afrequency lying in the passband of the filter when the capacitor is tobe charged.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch on and switch off the switch at afrequency lying in the stopband of the filter when the capacitor is tobe charged.

In accordance with various embodiments the state of the capacitor inwhich it is to be charged may correspond to a state in which the voltageacross the capacitor is smaller than the input voltage.

In accordance with various further embodiments of the circuit, theswitch may include a power switch.

In accordance with various further embodiments of the circuit, theswitch may be configured as a FET.

In accordance with various further embodiments of the circuit, theswitch may be configured as an NFET.

In accordance with various further embodiments of the circuit, theswitch may be configured as a PFET.

In accordance with various further embodiments of the circuit, theswitch may be configured as a current limiting element during thecharging of the capacitor.

According to various embodiments, a circuit is provided which mayinclude an input terminal to receive an input voltage, a first switch, afirst controlled terminal of which being coupled to the input terminal,an inductor, a first terminal of which may be coupled in series to asecond controlled terminal of the second switch, a freewheeling diode,wherein a first diode terminal may be coupled with the second controlledterminal of the first switch and with the first terminal of theinductor, and wherein a second diode terminal may be coupled with areference potential, a capacitor coupled with a second terminal of theinductor, a second switch, a first controlled terminal of which may becoupled to a second terminal of the inductor and a second controlledterminal of which may be coupled to a reference potential, and acontroller configured to operate the first switch, the second switch andthe inductor in continuous current mode to charge the capacitor.

In accordance with various further embodiments, the circuit may furtherinclude a battery, wherein the input terminal may be coupled to anoutput of the battery.

In accordance with various further embodiments of the circuit thecontroller may be configured to switch on the first switch synchronouslywith the second switch when the operation of the circuit is to beinitiated.

In accordance with various further embodiments of the circuit, thecontroller may be configured to switch off the first switchsynchronously with the second switch when the current through the anyone of the switches has reached a predetermined upper threshold value.

In accordance with various further embodiments of the circuit thecontroller may be configured to switch on the first switch synchronouslywith the second switch when the current through any of the switches hasfallen to a predetermined lower threshold value from a predeterminedupper threshold value.

In accordance with various further embodiments, the circuit may furtherinclude a sense resistor coupled between the second controlled terminalof the second switch and the reference potential.

In accordance with various further embodiments of the circuit thecontroller may be configured to determine the current flowing throughany of the switches by determining the current flowing through the senseresistor.

In accordance with various further embodiments of the circuit, thepredetermined lower threshold value is smaller than the predeterminedupper threshold value.

In accordance with various further embodiments, the circuit may furtherinclude a voltage sensing circuit configured to determine the voltageacross the capacitor.

In accordance with various further embodiments of the circuit thevoltage sensing circuit may be coupled to the controller.

In accordance with various further embodiments of the circuit thevoltage sensing circuit may be an integral part of the controller.

In accordance with various further embodiments of the circuit thecontroller may be configured to switch on the first switch permanentlywhen the voltage across the capacitor is substantially equal to theinput voltage.

In accordance with various further embodiments of the circuit thecontroller may be configured to operate the second switch and theinductor in continuous current mode as a boost converter when thevoltage across the capacitor is substantially equal to the inputvoltage.

In accordance with various further embodiments of the circuit as long asthe voltage across the capacitor is smaller than the input voltage, thecircuit may be configured to operate as a non-inverting buck-boostconverter.

In accordance with various further embodiments the circuit may furtherinclude a second capacitor which may be coupled in parallel to thecapacitor.

In accordance with various further embodiments of the circuit thecapacitance of the capacitor may be larger than 1 Millifarad.

In accordance with various further embodiments of the circuit thecapacitance of the capacitor may be larger than the capacitance of thesecond capacitor at least by a factor of 3.

In accordance with various further embodiments, the circuit may furtherinclude an input filter coupled to the input terminal

In accordance with various further embodiments of the circuit the filtermay be configured as a pi filter.

In accordance with various further embodiments of the circuit thecontroller may be configured to switch on and switch off the firstswitch synchronously with the second switch at a frequency lying in thepassband of the filter when the capacitor is to be charged.

In accordance with various further embodiments of the circuit thecontroller may be configured to switch on and switch off the switchsynchronously with the second switch at a frequency lying in thestopband of the filter when the capacitor is to be charged.

In accordance with various further embodiments of the circuit the firstswitch and/or the second switch may include a power switch.

In accordance with various further embodiments of the circuit the firstswitch and/or the second switch may be configured as a FET.

In accordance with various further embodiments of the circuit the firstswitch and/or the second switch may be configured as an NFET.

In accordance with various further embodiments of the circuit the firstswitch and/or the second switch may be configured as a PFET.

In accordance with various further embodiments of the circuit the firstswitch and/or the second switch may be configured as a current limitingelement during the charging of the capacitor.

In accordance with various further embodiments of the circuit the firstswitch and/or the second switch may be integrated in a common substrate.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

What is claimed is:
 1. A circuit, comprising: an input terminal toreceive an input voltage; a switch, a first controlled input of whichbeing coupled to the input terminal; an inductor, a first terminal ofwhich is coupled in series to a second controlled input of the switch; afreewheeling diode, wherein a first diode terminal is coupled with thesecond controlled input of the switch and with the first terminal of theinductor, and wherein a second diode terminal is coupled with areference potential; a capacitor coupled with a second terminal of theinductor; and a controller configured to operate the switch and theinductor in continuous current mode to charge the capacitor.
 2. Circuitof claim 1, further comprising: a battery, wherein the input terminal iscoupled to an output of the battery.
 3. Circuit of claim 1, wherein thecontroller is configured to switch on the switch when the operation ofthe circuit is to be initiated.
 4. Circuit of claim 1, wherein thecontroller is configured to switch off the switch when the currentthrough the switch has reached a predetermined upper threshold value. 5.Circuit of claim 1, wherein the controller is configured to switch onthe switch when the current through the switch has fallen to apredetermined lower threshold value from a predetermined upper thresholdvalue.
 6. Circuit of claim 1, further including: a voltage sensingcircuit configured to determine the voltage across the capacitor. 7.Circuit of claim 6, wherein the controller is configured to switch onthe switch permanently when the voltage across the capacitor issubstantially equal to the input voltage.
 8. Circuit of claim 6, whereinas long as the voltage across the capacitor is smaller than the inputvoltage, the circuit is configured to operate as a buck converter. 9.Circuit of claim 1, further comprising: at least one second capacitorwhich is coupled in parallel to the capacitor.
 10. Circuit of claim 9,wherein the capacitance of the capacitor is larger than the capacitanceof the second capacitor.
 11. Circuit of claim 1, further including: aninput filter coupled to the input terminal
 12. Circuit of claim 11,wherein the filter is configured as a pi filter.
 13. Circuit of claim11, wherein the controller is configured to switch on and switch off theswitch at a frequency lying in the passband of the filter when thecapacitor is to be charged.
 14. Circuit of claim 11, wherein thecontroller is configured to switch on and switch off the switch at afrequency lying in the stopband of the filter when the capacitor is tobe charged.
 15. Circuit of claim 1, wherein the switch comprises a powerswitch.
 16. Circuit of claim 1, wherein the switch is configured as acurrent limiting element during the charging of the capacitor.
 17. Acircuit, comprising: an input terminal to receive an input voltage; afirst switch, a first controlled terminal of which being coupled to theinput terminal; an inductor, a first terminal of which is coupled inseries to a second controlled terminal of the second switch; afreewheeling diode, wherein a first diode terminal is coupled with thesecond controlled terminal of the first switch and with the firstterminal of the inductor, and wherein a second diode terminal is coupledwith a reference potential; a capacitor coupled with a second terminalof the inductor; a second switch, a first controlled terminal of whichis coupled to a second terminal of the inductor and a second controlledterminal of which is coupled to a reference potential; and a controllerconfigured to operate the first switch, the second switch and theinductor in continuous current mode to charge the capacitor.
 18. Circuitof claim 17, further comprising: a battery, wherein the input terminalis coupled to an output of the battery.
 19. Circuit of claim 17, whereinthe controller is configured to switch on the first switch synchronouslywith the second switch when the operation of the circuit is to beinitiated.
 20. Circuit of claim 17, wherein the controller is configuredto switch off the first switch in sync with the second switch when thecurrent through the any one of the switches has reached a predeterminedupper threshold value.
 21. Circuit of claim 20, wherein the controlleris configured to switch on the first switch in sync with the secondswitch when the current through any of the switches has fallen to apredetermined lower threshold value from the predetermined upperthreshold value.
 22. Circuit of claim 17, wherein the circuit furthercomprises a sense resistor coupled between the second controlledterminal of the second switch and the reference potential.
 23. Circuitof claim 22, wherein the controller is configured to determine thecurrent flowing through any of the switches by determining the currentflowing through the sense resistor.
 24. Circuit of claim 17, furtherincluding: a voltage sensing circuit configured to determine the voltageacross the capacitor.
 25. Circuit of claim 17, wherein as long as thevoltage across the capacitor is smaller than the input voltage, thecircuit is configured to operate as a non-inverting buck-boostconverter.